This proposal tests a novel and innovative hypothesis in Rheumatoid Arthritis (RA) pathobiology. Specifically, we examine the role of complement C4B gene copy number variation (CNV) on RA pathogenesis and its interaction with HLA-DRB1 alleles (e.g. the shared epitope) in influencing RA phenotype (disease severity, B cell hyperactivity). Complement C4 proteins (C4A, C4B) are functionally distinguished by the nature of the covalent linkage they form with their targets: amino (C4A) vs thioester (C4B). Among their many essential roles, the acidic C4A and the basic C4B are important for the clearance of immune complexes. A deficiency of C4A is a well-established risk factor for human SLE, but the physiologic impact of C4B deficiency is under-studied. A diploid genome of a human subject contains zero to four copies of the C4B gene; serum levels of C4B strongly correlate with the number of C4B genes. In a RA population at Dartmouth, we observed that C4B deficiency (0-1 copy) is present in 43% of the seropositive patients, 31% of seronegative RA patients and 20% of non-RA patients or healthy controls (OR from 2.5 to 2.8). We hypothesize a particular role for C4B relative to C4A in RA mediated by its ability to clear immune complexes made up of anti-citrullinated protein antibodies (ACPA) and citrullinated antigens. We propose that the presence of deiminated-Arginines (i.e. citrullines with decreased numbers of amino groups) in these complexes results in a reduced ability of C4A to contribute to their clearance. As a result, the ability of the C4B-thioester carbonyl group to form a covalent linkage with a hydroxyl group of substrates for disposal makes C4B of much greater importance than C4A in the clearance of ACPA-immune complexes. Thus, deficient ACPA-based immune complex clearance associated with C4B deficiency would particularly enhance and favor maturation of anti-ACPA responses, promoting both seropositive RA and B cell hyperactivity. Hypothesis: We propose that C4B deficiency influences RA disease pathogenesis independent of the shared epitope (Aim 1). In Aim 2, we propose that C4B deficiency shapes RA phenotype and B cell hyperactivity (serum CXCL13, IgG and autoantibody levels).